Process to prepare a gas oil fraction and a residual base oil

ABSTRACT

The present invention provides a process to prepare a gas oil fraction, a heavy distillate fraction and a residual base oil fraction, which process at least comprises the following steps: (a) subjecting the feedstock to a hydroprocessing step to obtain an at least partially isomerised feedstock; (b) separating the isomerised feedstock by means of distillation into at least a gas oil fraction, a heavy distillate fraction and a residual fraction, wherein the residual fraction has a T10 wt % boiling point of between 200 and 450° C.; (c) recycling part of the residual fraction to step (a); and (d) catalytic dewaxing of remaining residual fraction to obtain a residual base oil.

The invention is directed to a process to prepare a gas oil and a base oil.

Such processes are known in the art, for example from WO2009080681. This publication describes a process to prepare a gas oil fraction and a residual base oil fraction from a Fischer-Tropsch derived feedstock, by subjecting the feedstock to a hydroprocessing step to obtain an at least partially isomerised feedstock; separating the isomerised feedstock by means of distillation into at least a gas oil fraction, a heavy distillate fraction and a residual fraction; recycling at least part of the heavy distillate fraction to hydroprocessing; and reducing the pour point of the residual fraction by means of catalytic dewaxing to obtain the base oil.

The process of WO2009080681 delivers a multitude of different products, however only a limited yield in heavy base oil. There also remains a general need in the art to further improve the yield and quality of Fischer-Tropsch products.

It is an object of the present invention to provide a process which can prepare at least a gas oil fraction and a residual base oil fraction. It is a further object of the invention to increase the quality and/or yield of the overall liquid fuel components.

From a first aspect, the invention resides in a process to prepare a gas oil fraction, a heavy distillate fraction and a residual base oil fraction from a Fischer-Tropsch derived feedstock, by (a) subjecting the feedstock to a hydroprocessing step to obtain an at least partially isomerised feedstock; (b) separating the isomerised feedstock by means of distillation into at least a gas oil fraction, a heavy distillate fraction and a residual fraction, wherein the residual fraction has a T10 wt. % boiling point of between 200 and 450° C.; (c) recycling part of the residual fraction to step (a); and (d) catalytic dewaxing of remaining residual fraction to obtain a residual base oil.

Applicants have found that with the process according to the invention a highly saturated residual base oil containing almost no sulphur and having a high viscosity index can be prepared. Furthermore a gas oil fraction is prepared with improved cold flow properties, and hence highly useful as a liquid fuel component. At the same time, the amount of heavy distillate is increased.

The Fischer-Tropsch derived feedstock is a feedstock produced in a Fischer-Tropsch condensation process. The Fischer-Tropsch condensation process is a reaction which converts carbon monoxide and hydrogen into longer chain, usually paraffinic, hydrocarbons in the presence of an appropriate catalyst and typically at elevated temperatures (e.g., 125 to 300° C., preferably 175 to 250 C) and/or pressures (e.g., 5 to 100 bar, preferably 12 to 70 bar). Other hydrogen to carbon monoxide ratios than 2:1 may be employed if desired.

The Fischer-Tropsch derived feedstock has preferably an initial boiling point of below 400° C. and a final boiling point of above 600° C. Preferably, the fraction boiling above 540° C. in the feedstock to step (a) is at least 20 wt %.

The hydrocracking/hydroisomerisation reaction of step a) is preferably performed in the presence of hydrogen and a catalyst, known to one skilled in the art as being suitable for this reaction. Catalysts for use in step (a) typically comprise an acidic functionality and a hydrogenation/dehydrogenation functionality. Preferred acidic functionalities are zeolites and refractory metal oxide carriers. Suitable carrier materials include zeolites, silica, alumina, silica-alumina, zirconia, titania and mixtures thereof. Zeolites may include Beta, Y, ZSM-12, EU-2, ZSM-48. Preferred carrier materials for inclusion in the catalyst for use in the process of this invention are silica, alumina and silica-alumina. A particularly preferred catalyst comprises platinum supported on a silica-alumina carrier. If desired, applying a halogen moiety, in particular fluorine, or a phosphorous moiety to the carrier, may enhance the acidity of the catalyst carrier.

Preferred hydrogenation/dehydrogenation functionalities are Group VIII noble metals, for example palladium and more preferably platinum. The catalyst may comprise the hydrogenation/dehydrogenation active component in an amount of from 0.005 to 5 parts by weight, preferably from 0.02 to 2 parts by weight, per 100 parts by weight of carrier material. A particularly preferred catalyst for use in the hydroconversion stage comprises platinum in an amount in the range of from 0.05 to 2 parts by weight, more preferably from 0.1 to 1 parts by weight, per 100 parts by weight of carrier material. The catalyst may also comprise a binder to enhance the strength of the catalyst. The binder can be non-acidic. Examples are clays, aluminas and other binders known to one skilled in the art. Examples of suitable hydrocracking/hydro-isomerisation processes and suitable catalysts are described in WO-A-0014179, EP-A-532118, EP-A-666894 and EP-A-776959.

The hydrocracking/hydroisomerisation reaction of step (a) is performed at elevated temperature and pressure. The temperatures typically will be in the range of from 175 to 380° C., preferably higher than 250° C. and more preferably from 300 to 370° C. The pressure will typically be in the range of from 10 to 250 bara and preferably between 20 and 80 bara. Hydrogen may be supplied at a gas hourly space velocity of from 100 to 10000 Nl/l/hr, preferably from 500 to 5000 Nl/l/hr. The hydrocarbon feed may be provided at a weight hourly space velocity of from 0.1 to 5 kg/l/hr, preferably higher than 0.5 kg/l/hr and more preferably lower than 2 kg/l/hr. The ratio of hydrogen to hydrocarbon feed may range from 100 to 5000 Nl/kg and is preferably from 250 to 2500 Nl/kg. The conversion in step (a) as defined as the weight percentage of the feed boiling above 370° C. which reacts per pass to a fraction boiling below 370° C., is at least 20 wt %, preferably at least 25 wt %, but preferably not more than 80 wt %, more preferably not more than 70 wt %. The feed as used above in the definition is the total hydrocarbon feed fed to step (a), including for example any recycle streams. The hydrocracked and at least partially isomerised feedstock obtained in step (a) may also be referred to as waxy raffinate. The waxy raffinate preferably has a relatively low pour point of below 40° C., more preferably below 35° C. and even more preferably below 30° C. The waxy raffinate preferably has a T10 wt % boiling point of between 200 and 450° C. and preferably between 300 and 420° C. The waxy raffinate may comprise the entire residual fraction of the atmospheric distillation. The waxy raffinate may have a T98 wt % recovery point of greater than 600° C.

In step (b) the feed is separated by means of distillation into at least a gas oil fraction, a heavy distillate fraction and a residual fraction. The distillation may be performed in one or more steps. The first step may be at atmospheric conditions, followed for example by a vacuum distillation. The distillation is suitably performed at low (vacuum) pressures, more preferably the vacuum distillation is performed at a pressure of between 1 and 250 mbar, more preferably between 10 and 100 mbar and most preferably between 10 and 75 mbar. Preferably the effective cutpoint temperature in step (b) at which the gas oil fraction and the higher boiling heavy distillate fraction are separated is between 300 and 400° C., and more preferably between 320 and 370° C. Preferably, the effective cutpoint temperature in step b) at which the heavy distillate fraction and the residual fraction are separated is at a temperature between 450 and 600° C.

The gas oil fraction will usually contain a majority (for instance 95 vol % or greater) of components having boiling points within the typical diesel fuel (“gas oil”) range, i.e., from about 150 to 400° C. or from 170 to 370° C. It will suitably have a 90 vol % distillation temperature of from 300 to 370° C. The gas oil will typically have a density (IP-365/97) from 0.76 to 0.79 g/cm³ at 15° C.; a cetane number (ASTM D-613) greater than 70, suitably from 74 to 85; a VK 40 (ASTM D-445) from 2 to 4.5, preferably from 2.5 to 4.0, more preferably from 2.9 to 3.7, centistokes; and a sulphur content (ASTM D-2622) of 5 mg/kg or less, preferably of 2 mg/kg or less.

The gas oil will suitably have a flash point (ASTM D-92) of 100° C. or higher, preferably 110° C. or higher, for example from 110 to 120° C.

The term gas oil fraction may refer to a middle distillate fraction suitable for producing gas oils and/or kerosene and/or diesel fuels.

The heavy distillate fraction will have an intermediate boiling range. Such a fraction preferably has a T90 wt % boiling point of between 400 and 550° C., preferably between 450 and 550° C. Optionally, at least part of the heavy distillate fraction may be recycled to step (a). For example more than 15 wt %, more than 30 wt %, or more than 40 wt %, or even more than 50 wt % of the heavy distillate fraction may be recycled to step (a). Typically at most 90 wt %, more preferably at most 80 wt %, even more preferably at most 70 wt % of the heavy distillate fraction may be recycled to step (a). In one embodiment of the invention, the whole heavy distillate fraction may be recycled to step (a).

The residual fraction is the bottoms fraction that remains after atmospheric and vacuum distillation. The 10 wt % recovery boiling point of said fraction is preferably above 400° C. and most preferably above 500° C. The residual fraction has a T10 wt. % boiling point of between 200 and 450° C., preferably between 300 and 240° C. T10 is the temperature corresponding to the atmospheric boiling point at which a cumulative amount of 10% of the product is recovered, determined using for example a gas chromatographic method such as ASTM D7169. In step (c) a part of the residual fraction is recycled to step (a). Preferably more than 20 wt %, more preferably more than 25 wt %, even more preferably more than 30 wt %, or yet more preferably more than 40 wt % of the residual fraction may be recycled to step (a). Typically at most 70 wt %, more preferably at most 60 wt %, even more preferably at most 50 wt % of the residual fraction may be recycled to step (a). An alternative would be for the whole residual fraction to be recycled to step (a).

The remaining residual fraction that is not recycled is dewaxed in step (d) to provide a residual base oil. Such dewaxing, or other dewaxing known in the art, may also be performed on the heavy distillate fraction to provide distillate base oils. Preferably more than 20 wt. %, more preferably more than 30 wt. %, and most preferably more than 40 wt. % and at most 60 wt. % of the residual fraction is catalytic dewaxded in step (d). Step (d) is performed by means of catalytic dewaxing. The catalytic dewaxing may be any process wherein in the presence of a catalyst and hydrogen the pour point of the base oil precursor fraction is reduced. Suitable dewaxing catalysts are heterogeneous catalysts comprising a molecular sieve and optionally in combination with a metal having a hydrogenation function, such as the Group VIII metals. Molecular sieves, and more suitably intermediate pore size zeolites, have shown a good catalytic ability to reduce the pour point of the base oil precursor fraction under catalytic dewaxing conditions. Preferably the intermediate pore size zeolites have a pore diameter of between 0.35 and 0.8 nm. Suitable intermediate pore size zeolites are mordenite, ZSM-5, ZSM-12, ZSM-22, ZSM-23, SSZ-32, ZSM-35, ZSM-48 or EU 2.

In the present invention, the reference to ZSM-48 and EU-2 is used to indicate that all zeolites can be used that belong to the ZSM-48 family of disordered structures also referred to as the *MRE family and described in the Catalog of Disorder in Zeolite Frameworks published in 2000 on behalf of the Structure Commission of the International Zeolite Assocation. Even if EU-2 would be considered to be different from ZSM-48, both ZSM-48 and EU-2 can be used in the present invention. Zeolites ZBM-30 and EU-11 resemble ZSM-48 closely and also are considered to be members of the zeolites whose structure belongs to the ZSM-48 family. In the present application, any reference to ZSM-48 zeolite also is a reference to ZBM-30 and EU-11 zeolite.

Besides ZSM-48 and/or EU-2 zeolite, further zeolites can be present in the catalyst composition especially if it is desired to modify its catalytic properties. It has been found that it can be advantageous to have present zeolite ZSM-12 which zeolite has been defined in the Database of Zeolite Structures published in 2007/2008 on behalf of the Structure Commission of the International Zeolite Assocation.

Another preferred group of molecular sieves are the silica-aluminaphosphate (SAPO) materials of which SAPO-Il is most preferred as for example described in U.S. Pat. No. 4,859,311. ZSM-5 may optionally be used in its HZSM-5 form in the absence of any Group VIII metal. The other molecular sieves are preferably used in combination with an added Group VIII metal. Suitable Group VIII metals are nickel, cobalt, platinum and palladium. Examples of possible combinations are Pt/ZSM-35, Ni/ZSM-5, Pt/ZSM-23, Pd/ZSM-23, Pt/ZSM-48 and Pt/SAPO-11. Further details and examples of suitable molecular sieves and dewaxing conditions are for example described in WO-A-9718278, U.S. Pat. No. 4,343,692, U.S. Pat. No. 5,053,373, U.S. Pat. No. 5,252,527 and U.S. Pat. No. 4,574,043.

The dewaxing catalyst suitably also comprises a binder. The binder can be a synthetic or naturally occurring (inorganic) substance, for example clay, silica and/or metal oxides. Natural occurring clays are for example the montmorillonite and kaolin families. The binder is preferably a porous binder material, for example a refractory oxide of which examples are: alumina, silica-alumina, silica-magnesia, silica-zirconia, silica-thoria, silica-beryllia, silica-titania as well as ternary compositions for example silica-alumina-thoria, silica-alumina-zirconia, silica-alumina-magnesia and silica-magnesia-zirconia. More preferably a low acidity refractory oxide binder material, which is essentially free of alumina, is used. Examples of these binder materials are silica, zirconia, titanium dioxide, germanium dioxide, boria and mixtures of two or more of these of which examples are listed above. The first preferred binder is silica. The second preferred binder is titania.

A preferred class of dewaxing catalysts comprise intermediate zeolite crystallites as described above and a low acidity refractory oxide binder material which is essentially free of alumina as described above, wherein the surface of the aluminosilicate zeolite crystallites has been modified by subjecting the aluminosilicate zeolite crystallites to a surface dealumination treatment. A preferred dealumination treatment is by contacting an extrudate of the binder and the zeolite with an aqueous solution of a fluorosilicate salt as described in for example U.S. Pat. No. 5,157,191 or WO-A-0029511. Examples of suitable dewaxing catalysts as described above are silica bound and dealuminated Pt/ZSM-5, silica bound and dealuminated Pt/ZSM-23, silica bound and dealuminated Pt/ZSM-12, silica bound and dealuminated Pt/ZSM-22, as for example described in WO-A-0029511 and EP-B-832171.

More preferably the molecular sieve is a MTW, MTT or TON type molecular sieve or ZSM-48 or EU-2, of which examples are described above, the Group VIII metal is platinum or palladium and the binder is silica.

Preferably the catalytic dewaxing of the residual fraction is performed in the presence of a catalyst as described above wherein the zeolite has at least one channel with pores formed by 12-member rings containing 12 oxygen atoms. Preferred zeolites having 12-member rings are of the MOR type, MTW type, FAU type, or of the BEA type (according to the framework type code). Preferably a MTW type, for example ZSM-12, zeolite is used. A preferred MTW type zeolite containing catalyst also comprises platinum or palladium metal as Group VIII metal and a silica or titania binder. More preferably the catalyst is a silica bound AHS treated Pt/ZSM-12 containing catalyst as described above. These 12-member ring type zeolite based catalysts are preferred because they have been found to be suitable to convert waxy paraffinic compounds to less waxy iso-paraffinic compounds. More preferably the above described catalyst comprising the 12-member ring zeolite is used in a first hydroconversion step to lower the pour point of the residual fraction to a intermediate value between the pour point of the feed and the pour point of the final base oil. More preferably the pour point of the intermediate product is between −10° C. to +10° C. The process conditions of such a first step may be suitably the catalytic dewaxing conditions as described below. This first hydroconversion step is followed by a final dewaxing step wherein preferably a catalyst is used which comprises a zeolite having at least one channel with pores formed by 10-member rings containing 10 oxygen atoms. Suitably as 10-member ring zeolites one of the following list comprising a TON type, MFI type, MTT type or FER type is used. The specific catalyst may be one as disclosed above which are according to these zeolite types. A preferred 10-member ring zeolite containing catalyst will also comprise a platinum or palladium metal as Group VIII metal and a silica binder. More preferably the catalyst is a silica bound AHS treated Pt/ZSM-5 or a silica bound AHS treated Pt/ZSM-23 containing catalyst as described above.

Catalytic dewaxing conditions are known in the art and typically involve operating temperatures in the range of from 200 to 500° C., suitably from 250 to 400° C., hydrogen pressures in the range of from 10 to 200 bar, preferably from 40 to 70 bar, weight hourly space velocities (WHSV) in the range of from 0.1 to 10 kg of oil per liter of catalyst per hour (kg/l/hr), suitably from 0.2 to 5 kg/l/hr, more suitably from 0.5 to 3 kg/l/hr and hydrogen to oil ratios in the range of from 100 to 2,000 liters of hydrogen per liter of oil. By varying the temperature between 275, suitably between 315 and 375° C. at between 40-70 bars, in the catalytic dewaxing step it is possible to prepare base oils having different pour point specifications varying from suitably +10° C. for the heavier grades to as far down to −60° C. for the lighter grades. The residual base oil may advantageously have a pour point of below 0° C., preferably below −30° C., more preferably below −50° C.

The residual base oil as obtained in step d) according to the process of the invention has preferably a kinematic viscosity at 100° C. (VK100) of above 15 cSt (15 mm²/s) as measured according to ASTM D-445. More preferably, the kinematic viscosity of the base oil of the invention at 100° C. (VK100) is at least 18 cSt, yet more preferably at least 21 cSt, again more preferably at least 23 cSt. For example, the base oil may have a VK100 in the range of from 15-35 cSt, preferably in the range of from 15-30 cSt, more preferably in the range of from 19-30 cSt, more preferably in the range of from 20-30 cSt, more preferably in the range of from 22-30 cSt, more preferably in the range of from 22-26 cSt, and most preferably in the range of from 24-26 cSt.

Kinematic viscosity described in this specification is determined according to ASTM D-445. The base oil as obtained in step d) according to the process of the invention contains preferably molecules having consecutive numbers of carbon atoms and preferably at least 95 wt % C30+ hydrocarbon molecules. More preferably, the base oil contains at least 75 wt % of C35+ hydrocarbon molecules.

“Cloud point” refers to the temperature at which a sample begins to develop a haze, as determined according to ASTM D-5773. The base oil typically has a cloud point between 80° C. and −60° C. Preferably, the base oil has a cloud point between 30° C. and −55° C., more preferably between 10° C. and −50° C. It was found that depending on the feed and the dewaxing conditions, some of the Fischer-Tropsch derived paraffinic heavy base oil would have a cloud point above ambient temperature, while other properties were not negatively affected.

“Pour point” refers to the temperature at which a base oil sample will begin to flow under carefully controlled conditions. The pour points referred to herein were determined according to ASTM D 97-93. Molecular weights were determined according to ASTM D-2503. Viscosity index (VI) is determined by using ASTM D-2270.

The residual base oil according to the subject invention preferably has a viscosity index of between 120-160. The residual preferably will contain no or very little sulphur and nitrogen containing compounds. This is typical for a product derived from a Fischer-Tropsch reaction, which uses synthesis gas containing almost no impurities. Preferably, the residual base oil comprises sulphur, nitrogen and metals in the form of hydrocarbon compounds containing in amounts of less than 50 ppmw, more preferably less than 20 ppmw, yet more preferably less than 10 ppmw. Most preferably it will comprise sulphur and nitrogen at levels generally below the detection limits, which are currently 5 ppm for sulphur and 1 ppm for nitrogen when using for instance by X-ray or Antek Nitrogen tests for determination. However, sulphur may be introduced through the use of sulphided hydrocracking/hydrodewaxing and/or sulphided catalytic dewaxing catalysts.

Furthermore, it was found that there appears to be a correlation between the kinematic viscosity, the pour point and the pour point depressing effect that an isomerised Fischer-Tropsch derived bottoms product could have. At a given feed composition and boiling range (as defined by the lower cut point from the distillate base oil and gas oil fractions after dewaxing) for the bottoms product, the pour point and the obtainable viscosity are linked to the severity of the dewaxing treatment. It was found that a pour point depressing effect was noticeable for isomerised Fischer-Tropsch derived bottoms products having a pour point of above −28° C. an average molecular weight between about 600 and about 1100 and an average degree of branching in the molecules between about 6.5 and about 10 alkyl branches per 100 carbon atoms as disclosed in U.S. Pat. No. 7,053,254. The Fischer-Tropsch derived residual base oil according to the invention can further be specified by its content of different carbon species. More particular, the Fischer-Tropsch derived residual base oil can be specified by the percentage of its epsilon methylene carbon atoms, i.e. the percentage of recurring methylene carbons which are four or more carbons removed from an end group and/or a branch (further referred to as CH2>4) as compared to the percentage of isopropyl carbon atoms. It was found that isomerised Fischer-Tropsch bottoms products as disclosed in U.S. Pat. No. 7,053,254 differ from the Fischer-Tropsch derived paraffinic base oil components obtained at a higher dewaxing severity in that the latter compounds have a ratio of percentages epsilon methylene carbon atoms to carbon atoms in isopropyl branches of at or above 8.2, as measured on the Fischer Tropsch base oil as a whole. It was found that a measurable pour point depressing effect through base stock blending as disclosed in U.S. Pat. No. 7,053,254 could only be achieved if in the base oil of the present invention, the ratio of percentages of epsilon methylene carbon atoms to carbon atoms in isopropyl branches was above or at 8.2. It is noted that where no pour point reducing effect is desired, the addition of a Fischer-Tropsch derived base oil having a lower pour point and higher ratio of compounds that have a ratio of percentages epsilon methylene carbon atoms to carbon atoms in isopropyl branches of at or above 8.2 may be beneficial, since such blends tend to be more homogeneous, as expressed by the lower cloud points. Therefore, preferably, the Fischer-Tropsch derived residual base oil according to the subject invention has a pour point of below −6° C., more preferably below −20° C. and most preferably below −28° C. Such a component (b) has no or only a negligible pour point depressing effect.

In a preferred embodiment of the current invention, the residual base oil obtained in step (d) is added to the gas oil fraction obtained in step (b). In only small amounts, preferably less than 5 wt % of base oil added to the gas oil, more preferably less than 3 wt % base oil, even more preferably less than 2 wt % of base oil, the quality of liquid fuel components of the gas oil improves even further.

The process as described above results in middle distillates having extremely good cold flow properties. For instance, the cloud point of any gas oil fraction is usually below −18° C., often even lower than −24° C. The CFPP is usually below −20° C., often −28° C. or lower. The pour point is usually below −18° C., often below −24° C.

The current invention is furthermore directed to the gas oil obtainable by the process of the current invention. The gas oil has a pour point of below −18° C., and a CFPP of below −20° C. This gas oil may be blended with a mineral gas oil to improve the quality of liquid fuel components of the mineral gas oil, as well as to lower the amount of sulphur in the mineral gas oil.

FIG. 1 schematically shows a process scheme of the process scheme of a preferred embodiment of the process according to the present invention.

For the purpose of this description, a single reference number will be assigned to a line as well as a stream carried in that line.

The process scheme is generally referred to with reference numeral 1.

In a Fischer-Tropsch process reactor 2 a Fischer-Tropsch product stream is obtained. This product 20 is fed to a hydrocracking/hydroisomerisation reactor 3 wherein part of it is converted to product boiling below a temperature in the range of 300 to 450° C. at atmospheric conditions. The effluent (not shown) of reactor 3 is distilled in a distillation column (not shown) to recover a middle distillates or gas oil fraction 30, a heavy distillates fraction 40 and a residual fraction 50. The middle distillates fraction 30 is distilled in a distillation column 4 to recover a gas oil 60 and kerosene 70. The heavy distillates fraction 40 is fed to a catalytic dewaxing reactor 5 to obtain a distillate base oil 80. The effluent 80 of reactor 5 is distilled in distillation column 6 to recover further base oils 90 with different kinematic viscosities at 100° C. between 2 and 10 mm²/s, preferably between 2 and 8 mm²/s. Part of the residual fraction 50 is fed to a catalytic dewaxing reactor 7 to obtain a residual base oil 100 with a kinematic viscosity at 100° C. between 15 and 35 mm²/s. Part of the residual fraction 50A is recycled to reactor 3 by combining 50A with the Fischer-Tropsch product 20. The present invention is described below with reference to the following Examples, which are not intended to limit the scope of the present invention in any way.

EXAMPLES

The C₅-C750° C.⁺ fraction of the Fischer-Tropsch product, as obtained in Example VII using the catalyst of Example III of WO-A-9934917, was continuously fed to a hydrocracking step (step (a). The feed contained about 60 wt % C₃₀+ product. The ratio C₆₀+/C₃₀+ was about 0.55. In the hydrocracking step the fraction was contacted with a hydrocracking catalyst of Example 1 of EP-A-532118. The effluent of step (a) was continuously distilled under vacuum to give a gas oil fraction, a heavy distillates fraction and a residual fraction. The yield and properties of the gas oil are given in Table 1. The yield is based on the feed to the hydrocracker. 60% of the residual fraction was recycled to step (a) and the remaining part was sent to a catalytic dewaxing step (d). The conditions in the hydrocracking step (a) were: a fresh feed Weight Hourly Space Velocity (WHSV) of 0.6 kg/l·h, recycle feed WHSV of 0.17 kg/l·h, hydrogen gas rate=750 Nl/kg, total pressure=77 bar, and a reactor temperature of 334° C.

In the dewaxing step, the residual fraction was contacted with a dealuminated silica bound ZSM-5 catalyst comprising 0.7% by weight Pt and 30 wt % ZSM-5 as described in Example 9 of WO-A-0029511. The dewaxing conditions were 40 bar hydrogen, WHSV=0.5 kg/l·h and a temperature of 320° C. The yield and properties of the obtained residual base oil are given in Table 1.

The heavy distillate fraction was subjected to a catalytic dewaxing step similar to the one described above. The obtained catalytic dewaxed oil was distilled into three base oil fractions boiling between 305 and 400° C., between 400-480° C. and a fraction boiling above 480° C. The yield and properties of the three distillate base oils are given in Table 1. The yield of the distillate base oils is based on liquid feed to catalytic dewaxing step.

TABLE 1 Viscosity Pour point (cSt) at Yield (%) (° C.) 100° C. Gas oil 38 −27 Naphtha 19 Distillate base oils 23 BO1 −35 3 BO2 −28 4 BO3 −24 8 Residual Base oil 19 −21 26

Comparative Example

Example 1 was repeated except that 50% of the heavy distillates fraction was recycled instead of the residual fraction. The yield and properties of the gas oil fraction, residual base oil and the distillate base oils are given in Table 2.

TABLE 2 Viscosity Pour point (cSt at Yield (%) (° C.) 100° C.) Gas oil 35 −30 Naphtha 22 Distillate base oils 10 BO1 −35 3 BO2 −28 4 BO3 −24 8 Residual Base oil 31 −21 26

Discussion

It is shown in Tables 1 and 2 that by comparing Comparative Example where the heavy distillates fraction is partly recycled to the hydrocracking/hydroisomerisation step with Example 1 according to the present invention where the residual fraction is partly recycled, that the overall liquid yield improved, with a higher amount of gas oil, the amount of distillate base oils improved, while still a sizeable fraction of residual base oil was produced. In addition, the amount of less attractive naphtha was reduced. 

1. A process to prepare a gas oil fraction, a heavy distillate fraction and a residual base oil fraction from a Fischer-Tropsch derived feedstock, by (a) subjecting the feedstock to a hydroprocessing step to obtain an at least partially isomerised feedstock; (b) separating the isomerised feedstock by means of distillation into at least a gas oil fraction, a heavy distillate fraction and a residual fraction, wherein the residual fraction has a T10 wt. % boiling point of between 200 and 450° C.; (c) recycling part of the residual fraction to step (a); and (d) catalytic dewaxing of remaining residual fraction to obtain a residual base oil.
 2. A process Process according to claim 1, wherein the gas oil fraction and the heavy distillate fraction are separated in step (b) at a cut point temperature of between 300 and 400° C.
 3. A process according to claim 1, wherein the heavy distillate fraction and the residual fraction are separated in step (b) at a cut point temperature of between 450 and 600° C.
 4. A process according to claim 1, wherein the Fischer-Tropsch derived feedstock has an initial boiling point of below 400° C. and a final boiling point above 600° C.
 5. A process according to claim 1, wherein the fraction boiling above 540° C. in the feedstock to step (a) is at least 20 wt %.
 6. A process according to claim 1, wherein the residual base oil obtained in step (d) has a kinematic viscosity at 100° C. according to ASTM D-445 in the range of from 15-35 cSt.
 7. A process according to claim 1, wherein the base oil obtained in step (d) has a cloud point of between 80° C. and −60° C. as measured according to ASTM D-5773.
 8. A process according to claim 1, wherein the base oil obtained in step (d) has a pour point of below 0° C. as measured according to ASTM D-97-93.
 9. A process according to claim 1, wherein the base oil as obtained in step (d) has a Viscosity Index of between 120 and 160 as measured according to ASTM D-2270.
 10. A process according to claim 1, wherein the base oil obtained in step (d) is added to the gas oil fraction obtained in step (b).
 11. A process according to claim 1, wherein the dewaxing step is performed by means of a catalytic dewaxing process in the presence of a catalyst comprising a medium pore size molecular sieve and a group VIII metal.
 12. A process according to claim 11, wherein the molecular sieve is a MTW, MTT, TON type molecular sieve or ZSM-48 or EU-2.
 13. A process according to claim 11, wherein the Group VIII metal is platinum or palladium.
 14. A process according to claim 11, wherein the catalyst used in the catalytic dewaxing of the residual fraction comprises a MTW molecular sieve, platinum or palladium as Group VIII metal and a silica or titania binder. 